12.4
Dynamic Characteristic and Contact Bounce
Analysis for an AC Contactor with PWM
Controlled Coil
Yingyi Liu, Degui Chen, Liang Ji, Yingsan Geng
State Key Laboratory of Electric Insulation and Power Equipment
Xi'an Jiaotong University
Xi'an, China
dgchen@,mail.xjtu.edu.cn
In the aspect of dynamic simulation, previous studies [4]-[9]
built different mathematic models to compute the dynamic
behaviors of contactors, but few scholars did research on the
dynamic simulation of the contact bounce. In [10]-[11], the
authors studied the contact bounce of the general AC contactor
with uniform magnetic field. And up to now, little attention is
paid to the dynamic characteristic simulation of contactors
considering the contact bounce with PWM controlled coil.
Abstract-In the optimum design of AC contactors, it is
important to analyze the dynamic behavior. Moreover, movable
contact and core bounces have remarkable effect on the lifetime
of contactors. According to a kind of intelligent contactor with
feedback mechanism, this paper builds two different sets of
periodically inter-transferred equations. The equations describe
the coupling of the electric circuit, electromagnetic field and
mechanical system taking account of the influence of friction.
And with virtual prototyping technology, the dynamic behavior
of the contactor can be obtained. The paper introduces the
contact resistance to investigate the bounce of the contact.
F
Keywords- intelligent contactor; dynamic characteristics;
contact bounce; contact resistance.
I.
INTRODUCTION
The contactor is most widely used in all low-voltage
apparatus. In recent years, the number of contactors in use is
soaring along with the development of the industry control, so
the issue that how to lengthen the contactor's lifespan and
make it energy-saving has become the important research
project in improving the contactor's capability. Therefore, the
fact that the bounce of the contact greatly affects the lifespan
of the contactor has attracts a number of researches on it.
In [1]-[2], the author decreased the contact velocity by
controlling and restricting the electrified time of the coil. In
[3], Stefan Jorgens researched the most important influencing
factors on contact velocity and bounce time. Meanwhile, many
corporations put their intelligent contactors into the market.
For example, the Westinghouse Advantage contactor selects
the optimal dynamic characteristic curve of the contactor by
detecting the current of the coil. Once the contacts are closed,
the coil current is decreased rapidly to reduce the velocity of
the movable core at the instant that it impacts with the static
core. Consequently, the second bounce of the contacts is
reduced greatly. Another example is the DIL-M series
intelligent contactors of Moeller Corporation, which adjusts
the current of the coil according to the optimal attractive force
curve storing in the microcomputer chip to cooperate with the
load force curve best.
1-4244-0838-5/07/$25.00 02007 IEEE
2
Fig. 1 Cooperative curves of the attractive force and load force
UA k
Ue
L UZ
-
pick-up process
contactor has picked up
Fig. 2 Coil voltage with feedback control
289
t
In this paper, an intelligent contactor with a voltage
feedback system is considered. Fig 1 shows its cooperative
curves of the attractive force and load force, where curve 1
represents the load force and curve 2 represents the attractive
force.
In the pick-up process of the intelligent contactor, it
keeps the coil voltage unchanged by its voltage feedback
system when the voltage fluctuation is between 85%0 110%.
As shown in Fig 1, the corresponding attractive force curve is
slightly higher than the load force curve. Fig 2 shows the
changing principle of the coil voltage with feedback control,
where U, is the voltage during the pick-up process of the
contactor, and Ub is voltage after the contactor has picked up.
It is obvious that Ub is lower than U, greatly, so the energy
consumption of the contactor when it has picked up is very
low.
The function of the voltage feedback system described
above is achieved by using a single microcomputer chip based
circuit. Fig 3 demonstrates the block diagram of the circuit,
where the rectified AC input is fed to the coil through a PWMI
modulator. The PWM modulator is controlled by a
microprocessor-based controlling module, which receives the
feedback signal of the voltage to decide the duty ratio of the
PWM modulator with an IGBT. In order to keep the supply
voltage of the coil unchanged, it allocates different duty ratios
of the modulator to correspond with different external
voltages. Fig 4 shows the modulated coil voltage when the
supply voltage is 220V and the modulating frequency of PWM
is 5000HZ.
rectification circuit
PWM modulator
AC~1
et
ct
soc
mentioned above, the paper uses the virtual prototype software
ADAMS to study the dynamic characteristics of the intelligent
contactor, especially the bounce characteristic of the contact
with contact resistance. In order to calculate the coupling
equations of the mechanical movement, magnetic field and
electrical circuit, a secondary development is carried out for
ADAMS. Meanwhile, the validation is done for the simulation
result with experiment.
II.
ANALYZED MODEL AND METHOD
A. Analyzed Model
In this paper, an intelligent contactor rated 160A is studied.
Fig 5 shows its simplified model in ADAMS. Here, we equate
6 springs of the contactor to 1 equivalent spring according to
the equivalent principle of ADAMS.
contact spring
movable contact
core
static core
ci
wc
controlling block
feedback
voltageblock
Fig. 5 Analyzed model
B. Mathematic Model
In the aspect of electric circuit, ignore the eddy current of
the core then the electrical behavior of the coil can be
Fig. 3 Block diagram of the feedback system
>300
described by
d wf =
dt
d VI =-iR
-Ur sin(wt + p) -iR1
100
0.0
0.005
t/s
(1)
(2)
R: resistance of the coil
/2: flux linkage of the coil
d 1dt: change rate of flux linkage, i.e. the inverse voltage of
the coil
Urn: peak value of AC voltage
|Ur sin(ca +p) absolute value ofthe AC voltage
Fig. 4 Waveform of the coil voltage
Consequently, with the help of the feedback system, the
intelligent contactor can greatly reduce the kinetic energy of
the contactor's movable parts during its pick-up process. Thus
reduce the bounce of contact and increase the lifespan of the
contactor. Moreover, by keeping a lower voltage on the coil
after the contactor has picked up, the energy consumption of
the contactor can be reduced.
According to the characteristics of the intelligent contactor
(): angular frequency
(0: switching phase angle
i : current of the coil
When the voltage of the coil corresponds to the high level
290
displacement value xl and transfers it again to the user
program to get next force value F2. Repeat the above cycles
until the whole dynamic behavior including the contact bounce
of the contactor is obtained.
of the impulse wave, i.e. the closing time of IGBT, the
electrical behavior of the coil can be represented by equation
(1). And equation (2) represents the electrical behavior of the
coil when IGBT is open.
In the aspect of mechanism, the contactor satisfies the
Newton-Euler variable differential equation (3) in its pick-up
process.
&rT[m? - F] + &rT[j'c' + eoJ'w' - n'] = 0
(3)
Sr: virtual displacement
i)r': virtual rotary movement
m mass of the object
F total external force acting on the object
n': total torque of the external force relative to the grid
C. Calculation Methodfor contact bounce
In the pick-up process of the contactor, the movable contact
moves to impact with the static contact and causes the first
bounce. After the over travel, the movable core impacts with
the static core and causes the second bounce of the contact. So
the total bounce time is the sum of the first and the second
bounce time.
In this paper, a contact resistance is introduced to simulate
the bounce of the contact. The relationship between the
contact resistance and contact pressure can be written as [12]
(8)
Rcc(FP) 1
Where R is the contact resistance, Fp is the contact pressure.
During the process of contact bounce, the contact pressure
changes with the deformation of the spring, and the contact
resistance changes correspondingly according to expression
(8).
The calculation of the contact bounce simulates its actual
measuring method. Firstly, according to the displacement x,
calculate the contact pressure Fp and consequently the value of
the contact resistance R. With the voltage of the sampling
resistance, the contact bounce wave can be obtained with
equation (9).
origin
J': constant inertial matrix
r: acceleration of the object
of: angular velocity
C: vector angular velocity
co: angular acceleration
Combining with the electrical behavior of the coil, the
dynamic behavior of the contactor can be written as:
dV
(4)
+
-i1R1
4
{dt =1 Um sin(t )
co+ cJ'' - n'] = 0
arT[m? - F] +
ddt -i1R1
{ cT[=4R1
co+ OJ'' - n'] = O
-F]+
Where the two equations is used periodically according to
the on-off state of IGBT.
For the given exciting current i1 and displacement x, the
values of qvand F are defined as
=
f1(i4,x)
f2(il, x)
R
R=
(9)
Where Uc is the voltage of the sampling resistance, U1=lV
(U1 accounts for the voltage of DC current source), R is the
contact resistance, which is infinite when the contactor is
open, Rc=1Q ( Rc is the sampling resistance ).
(6)
(7)
III.
F=
Where f4 (i1, x) , f2 (il, x) are two specific transfer
functions of the contactor. They are one-to-one corresponding
to the variables i1, x. But up to now, their expressions are still
undecided. In this paper, two data sheets describing the
correspondences between tvto i1 , x and F to i1, x are built.
SIMULATION RESULT AND EXPERIMENT
A. Current Simulation and Experiment Validation
With the method in II, the simulation current of the coil in
the contactor's pick-up process can be obtained. Fig 6 shows
the simulation result of current when the voltage is 220V and
the switching angle is 00.
4r
3
And the values of qvand F at any combination of i1, x can be
obtained with interpolation method.
In the calculation of equations (4) and (5), the mechanical
equation is calculated by the ADAMS program, and the
coupling equations of the mechanical movement, magnetic
field and electrical circuit are calculated by the user program.
The user program and ADAMS cooperate with each other in
an interactive mode. Firstly, the ADAMS program transfers
the displacement value xo to the user program, with which the
user program calculates the force value F1 and returns it to
ADAMS. Then ADAMS uses F1 to calculate next
¢2
I
o
1'
0.00
0.02
0.04
tls
Fig. 6 Simulated result of current
The experiment installation for measuring the current wave
of the coil is as shown in Fig 7, where the AC source is
291
switched on with a phase selector, and the current wave of the
coil is obtained indirectly by measuring the voltage wave of a
small resistance connected with the coil in series.
phase ||
selector r|
Time: 0.054
-Currert -0.344
E 250
t me
CD
rD1
C
AC
500
oscillograph|
sampling
o
collision
-1000
0..01
0.0275
0.055 t/s
Fig. 9 Speed of movable contactors
Fig. 7 Schematic diagram of experiment
Fig 8 is the experimental current wave of the coil. By
comparison between Fig. 6 and Fig. 8, it indicates that the
simulated result agrees with the experimental result well. The
maximum experimental current of the coil is 3.2A, while the
calculated result is 3.45A, so the error percentage doesn't
exceed IO0%.
4.0
1
2
3
4
5
1 electromagnet; 2 connecting steel wire; 3 displacement sensor
bracket; 4 displacement sensor; 5 fixed base; 6 connecting nut
A
Fig. 10 Experimental installation for displacement-measurement
3.0
:
The experimental result of the contact's closing velocity is
0.58m/s, while the simulated result is 0.63m/s. The difference
between the two results is within the acceptable range.
C. Contact Bounce Simulation and Experiment Validation
2.0
1.0
VI
_
-iw
P,
V.v
0.01
0.03
0.05
TABLE I.
t/S
Stiffness
Fig. 8 Experimental result of current
(N/mm)
Ix105
B. Contact Velocity Simulation and Experiment Validation
Fig 9 shows the simulation result of the contact velocity
when the coil voltage is 220V and switching angle is 00. The
simulated closing velocity is 0.63m/s.
MATERIAL PARAMETERS OF THE CONTACTOR
Nonlinear
factor of
force
1.5
Maximum viscous
damping factor
(N.S I mm)
1.0
Deforming extent
with maximum
damp (mm)
0.1
With the method in II, the simulation contact bounce can be
obtained. The contact parameters needed in simulation are
listed in Tab. 1, where the stiffness and nonlinear factor of
force are obtained by looking up table, and the maximum
viscous damping factor and deforming extent with maximum
damp are obtained by experiment. The parameters of stiffness,
nonlinear factor of force and deforming extent with maximum
damp hardly affect the bounce time of the contacts. However,
the bounce time of the contacts increases obviously when the
maximum viscous damping factor decreases.
Fig 11 shows the simulated result of contact bounce when
the voltage is 220V and the switching angle is 00. The
simulated bounce time is 1. lIms.
The following method is applied for measuring the contact
bounce: Connect a DC current source across the contact
through a resistance, and capture the voltage wave of the
resistance to get the bounce condition of the movable contact
during its pick-up process. Fig. 12 is the experimental bounce
wave of the contact when the voltage is 220V and the
switching angle is 00. The average bounce time of several
The following experimental idea is used to measure the
closing velocity of the contact: Firstly, capture a small section
of displacement wave near the closing point of the contactor.
Then differentiate the displacement wave to get the closing
velocity of the contact. According to this idea, an experimental
installation is built as shown in Fig 10 where the displacement
of the electromagnet is passed to the displacement sensor
through an inflexible connecting wire. The output of the
displacement sensor is a voltage wave, which can be converted
to displacement wave by some conversation factor of the
sensor. Connect a DC source across the contact through a
resistance, and capture the voltage wave of the resistance as
well as the displacement wave of the displacement sensor at
the same time. When the contact is closed at time t, a kick will
occur in the voltage wave of the resistance. Differentiate the
displacement wave at time t, thus the closing velocity of the
contact is obtained.
292
experimental results is 1.Oms, which is quite near the
simulated result.
characteristics of the contactor, the contact bounce in the
pick-up process ofthe contactor can be simulated.
References
[1]
1.2 r
0.8
-
0.4
-
[2]
[3]
0.0
0.0495
1II
,,
0.0505
0.0515
tls
[4]
Fig. 11 Simulated result of contact bounce
[5]
11
0.
o.o 1L,
0.0450
,
0.0505
[6]
0.0515
0.0525
[7]
t/s
Fig. 12 Experimental result of contact bounce
[8]
CONCLUSIONS
(1) This paper puts forward a method in calculating the
dynamic characteristics of the intelligent contactor with
voltage feedback system. It builds two periodically interconverted equations sets to describe the dynamic behavior
of the contactor. In order to calculate the coupling
equations of the mechanical movement, magnetic field
and electrical circuit of the contactor, a user program is
developed in association with ADAMS program. With the
interaction of the two programs, the whole dynamic
characteristics of the contactor can be calculated. The
experiment results validate the correctness of the
simulation results.
(2) Influence of the contact resistance is considered for
contact bounce [12]. Combing with the dynamic
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